AbstractElectrochemical CO₂ reduction (CO₂R) to ethylene and ethanol enables the long-term storage of renewable electricity in valuable multi-carbon (C₂₊) chemicals. However, carbon–carbon (C–C) coupling, the rate-determining step in CO₂R to C₂₊ conversion, has low efficiency and poor stability, especially in acid conditions. Here we find that, through alloying strategies, neighbouring binary sites enable asymmetric CO binding energies to promote CO₂-to-C₂₊ electroreduction beyond the scaling-relation-determined activity limits on single-metal surfaces. We fabricate experimentally a series of Zn incorporated Cu catalysts that show increased asymmetric CO* binding and surface CO* coverage for fast C–C coupling and the consequent hydrogenation under electrochemical reduction conditions. Further optimization of the reaction environment at nanointerfaces suppresses hydrogen evolution and improves CO₂ utilization under acidic conditions. We achieve, as a result, a high 31 ± 2% single-pass CO₂-to-C₂₊ yield in a mild-acid pH 4 electrolyte with >80% single-pass CO₂ utilization efficiency. In a single CO₂R flow cell electrolyzer, we realize a combined performance of 91 ± 2% C₂₊ Faradaic efficiency with notable 73 ± 2% ethylene Faradaic efficiency, 31 ± 2% full-cell C₂₊ energy efficiency, and 24 ± 1% single-pass CO₂ conversion at a commercially relevant current density of 150 mA cm⁻² over 150 h.